Add tempo command to work with timed notes

[rbnpi]

The with_tempo command is a good idea but it doesn't apply to timed durations
It would be nice to have a tempo multiplier command to apply to timed sequences. Otherwise ALL times have to be recoded which is too time consuming

[samaaron]

In version 2.0 you can use with_bpm to apply block-level modifications to times:

with_bpm 120 do
  play_pattern_timed [chord(:e5, :minor), chord(:f4, :minor), chord(:gb, :major)], [1, 0.5]
end